In the spring of 1979, two water company employees were found dead at the bottom of a water-filled underground meter vault. A month-long investigation disclosed that the atmosphere of the vault had progressively been deteriorated by the growth of primitive plants present in the water and also on the walls and ceiling of the vault. When entering manholes, employees must adhere strictly to all safety rules, such as the testing of the air before entry, ventilation of the air spaces, and the presence of a standby rescue worker. The diagnosis of acute stagnant air syndrome for this type of accident is introduced herein, to alert industrial concerns and the medical professions about the existence of this occupational hazard confronting individuals who work in confined and poorly ventilated manholes.